Kona, if you have switched the speakers and it is still on the same channel and have changed amps and it is still on the same channel, then the problem is coming from before the amp. From your description, it is a subsonic frequency. When you used a different pair of speakers, they may not respond to a frequency that low and made you think that they weren't having the problem. Disconnect the preamp from the amp's inputs and see if that stops it. I think it's in the preamp from what you've described. Since it happened when the volume was all the way down, that pretty well eliminated any source problems. When you changed amps, that eliminated the amp. When you switched speakers and the problem remained on the same channel, that eliminated the speaker. The preamp is the only thing left. Try a different preamp.
tube amp/flutter in speakers?
i have a nice pair of mono blks that are making my woofers flutter. when i adjust the hum bal it gets stronger or softer depending on which way i turn the adj. my interconnects are good. i am using an old pair of ess monitors with the heil driver. the amps are sansui HF V60. rated at 15wpc i belive?. thanks to all who help.